code: def flatten(data):
      return np.reshape(data, [data.shape[0], -1])
  
file_path: MONAI/tests/test_handler_regression_metrics.py
imports: from __future__ import annotations
  import unittest
  from functools import partial
  import numpy
  import torch
  from ignite.engine import Engine
  from monai.handlers import MeanAbsoluteError, MeanSquaredError, PeakSignalToNoiseRatio, RootMeanSquaredError
  from monai.utils import set_determinism
label_desc: Use image node_id 1 for calling a global function with example usage: flatten(data) and returns: np